Iece apparatus for removing material from an electrically conducting workp

ABSTRACT

Spark erosion machine including an electrode holder driven by a motor. A control element in the form of an electromechanical transducer is arranged within the electrode holder, permitting fast fine control of the electrode movement.

United States Patent 1 1 De Jongh et al.

1 1 Sept. 11, 1973 APPARATUS FOR REMOVING MATERIAL FROM AN ELECTRICALLYCONDUCTING WORKPIECE [75] Inventors: Job Harm De Jongh; JohannesJosephus Aloisius Cornelus Maenhout; Cornelis Van Osenhruggen, all ofEmmasingel, Eindhoven, Netherlands I73] Assignee: U.S. PhilipsCorporation, New York,

22 1 Filed: Oct. 10, 1972 21 1 Appl. No.: 295,946

[ 30] Foreign Application Priority Data Oct. 13, 1971 Netherlands...'.....,7114o4s 52 US. Cl. 219/69 G, 204/143 M, 219/69 v 51 Int. Cl 823p1/04, 323p 1/14 [58] Field of Search 219/69 G, 69 V; 204/143 M [56]References Cited UNITED STATES PATENTS 3,335,313 8/l967 Luimes et al.219/69 G X 3,610,865 l0/l97l Van Osenbruggen 219/69 G X PrimaryExaminer-R. F. Staubly AttorneyFrank R. Trifari [57] ABSTRACT Sparkerosion machine including an electrode holder driven by a motor. Acontrol element in the form of an electromechanical transducer isarranged within the electrode holder, permitting fast fine control ofthe electrode movement.

4 Claims, 2 Drawing Figures 2 l/ 1 I 1 2 I \4 PAIENTEUSEPI 1 ma sum '1or 2 I APPARATUS FOR REMOVING MATERIAL FROM AN ELECTRICALLY CONDUCTINGWORKPIECE The invention relates to an apparatus for removing 7 materialfrom an electrically conducting workpiece by means of spark erosion inwhich sparks are produced between the workpiece and an electrode, whichapparatus includes an electrode holder driven by a motor. Such anapparatus is described in Philips Technical Review, 30 No. 6/7, pages195-208 (1969). The high-speed spark machine described on page 202 ofthis paper has the advantage that the repetition frequency of the sparkdischarges in the spark gap, i.e., the space between the work piece andthe electrode, is high, permitting the spark machining rate to be highalso. In the known apparatus a stepping motor is controlled by thebreakdown voltage across the spark gap. The stepping motor may drive theelectrode holder, for example, via a lead screw. After each sparkdischarge the electrode holder is displaced a small distance.

The known apparatus using discontinuous electrode displacement is notsuitable for very fine machining operations. Driving the electrode bymeans of a stepping motor cannot be used to form very small holes inaworkpiece, which operation requires the use of a small spark energy anda correspondingly narrow spark gap, because the discretesteppeddisplacement is greater than the desired spark gap. Y

It is an object of the invention to provide an apparatus of the typedescribed at the beginning of this specification which not only worksfast but also is suitable to perform very fine operations. The apparatusaccording to the invention is characterized in that the electrode holderincludes a control element for the electrode in the form of anelectromechanical transducer which similarly to the motor is controlledby a signal derived from the condition of the spark gap. Theelectromechanical transducer ensures very fast fine control, theelectrode being spaced from the workpiece by a dis tance which isfavourable for the spark machining process. At a given signal strength amotor, for example a stepping motor, is 'turned on so that the electrodeholder is moved a given distance. During the step displacement theelectrode is held at the required distance from the workpiece by thefast electromechanical control.

The invention is not restricted to a spark erosion machine using astepping motor. An electromechanical transducer may also be used toproduce fine control of th electrode displacement when the electrode isdriven by means of a servomotor.

Preferably the control element is a piezoelectric element which isclamped at both ends and comprises a plurality of plates made of apiezo-electric material which are provided with electrically conductingcoatings and are arranged one above the other in the direction of theelectrode movement.

It should be noted that the use of piezo-electric elements in sparkerosion machines is known. For example, on pages 204 to 206 of "PhilipsTechnical Review," 30 No. 6/7 (1969) there is described a spark erosionmachine in which the electrode is vibrated by means of a piezo-electricelement. In this machine,

however, the piezo-electric element is clamped at one vided withelectrically conducting coatings. The mechanical stiffness of the knowndevice is small, so that the control frequency is comparatively low. Thedevice according to the invention has a large mechanical stiffness and acontrol frequency which is an order of magnitude higher than that of theknown apparatus. In the known apparatus the piezo-electric element isdirectly controlled by the voltage across the spark gap, whereas in theapparatus according to the invention the control element is controlledby a signal obtained by comparing the current flowing, and/or thevoltage applied, across the spark gap with reference values.

In the apparatus according to the invention a hydraulic amplitudeamplifier is preferably provided between the control element and theelectrode. This enables the amplitude of the control signal to beconsiderably reduced.

An embodiment of the invention will now be described, by way of example,with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 shows schematically a preferred embodiment of an apparatusaccording to the invention and FIG. 2 shows in more detail the electrodeholder used in this apparatus.

In the apparatus shown in FIG. 1 a workpiece W is held by a mount 1. Theworkpiece is placed in a tank 2 containing a dielectric rinsing liquid3. The tank 2 is placed on a worktable 4. A work electrode EL is securedin an electrode holder 6 which in turn is rigidly connected to a movablecolumn 7. The column 7 can be raised and lowered by means of a leadscrew 8 driven by a stepping motor 9 within a guidecolumn 5 connected toa column 4 of the worktable.

Sparks between the electrode and the workpiece are produced by means ofa pulse generator 10 which supplies well defined rectangular currentpulses.

The displacement of the electrode EL is to be controlled in accordancewith the progress of the machining process. For this purpose a device 11is provided which measures the condition'of the spark gap and convertsthe measurement into a control voltage, preferably a direct voltage. Aswill be set out more fully hereinafter, the device 11 may be based onvarious principles. The control voltage at the output of the device 11is applied via a voltage amplifier 12 to a control element, for examplea piezo-electric element, in the electrode holder 6.

FIG. 2 shows this electrode holder. A housing 20 contains apiezo-electric control element 21 which comprises a plurality of plates22 made of a ceramic material which has a given bias polarisation. Theplates 22 are separated by electrically conducting coatings 23 which arealternately connected to terminals 24and 24. The bias polarisation ofthe plates 22 will be reduced in accordance with the voltage applied tothe terminals 24 and 24', causing the element 21 to be deflected to agreater or lesser extent, so that the electrode EL also is given avoltage-dependent displacement. a t

As soon as the voltage across the piezo-electric element exceeds a givenvalue, there is applied via a level detector 13,.which is indicated inFIG. 1 and may include a differential amplifier, a signalto the steppingmotor, causing it to displace the electrode holder a given distance.Thepiezo-electric control is so fast as to enable the .distancebetweenthe electrode and the workpiece to be adjusted within the duration ofthe step displacement of the motor.

In a spark machining process three conditions can be distinguished:

a. The desired condition in which material is removed from theworkpiece. During this condition there is a given voltage, the breakdownvoltage, at which a spark is struck between the workpiece and theelectrode. After a breakdown a current flows across the spark gap.

b. The condition in which, although there is a voltage across the sparkgap, no current flows.

. c. The short-circuit condition, during which a large current flows butno voltage is set up between the workpiece and the electrode.

In the two latter conditions no material is removed and the electrode isto be displaced, in the case b towards the workpiece and in the case caway from the workpiece.

Because each of the aforementioned conditions is characterizedby thepresence or absence of current flowing and voltage applied across thespark gap or by the value of this current or voltage, measuring at leastone of these values and comparing the or each meausred value with arelevant reference value enables a control voltage to be produced whichgives an indication of the condition of the spark gap. 7

For example, the breakdown voltage between the electrode and theworkpiece may be measured, the average of the breakdown voltage beingcompared with a reference direct voltage. The electrode can then becontrolled by means of the difference between the average breakdownvoltage and the reference voltage.

A more accurate method is theso-called efflciency determination in whichboth the current flowing and the voltage applied across the dischargespace are measured and processed in a logic circuit. Such a method isdescribed in the paper: "Entladeund Abtragvorgange sowieVorschubregelung bei der Elektroerosion" (Discharge and removalprocesses and feed control in electro-erosion") by W. Weigand in"Industrie- Anzeiger," 30 (1969), No. 42, pages 41-44.

The piezo-electric control element may also'have the shape of a hollowcylinder. The electrically conducting coatings then are provided on theinner and outer walls of the cylinder. The axial deflection per unit offield strength of such a piezo-electric cylinder is small. However, thecylinder wall may be thin, permitting a high field strength to beobtained with a not too high control voltage.

The aforedescribed apparatus employing a piezoelectric control elementis only one embodiment of an apparatus according to the invention. Thecontrol element may be any electromechanical transducer, provided thatit is fast enough and gives the desired deflection. For example, thepiezo-electric control element of FIG. I may be replaced by amagnetostrictive element which is controlled by a current derived fromthe conv dition of the spark gap.

The control element (FIG. 2) may be directly connected to the rod 30 andthe electrode EL. For such a construction comparatively large controlsignals (for example control voltages up to 1,500 volts) are required todisplace the electrode through the desired distances. Especially theamplification of direct voltages to such values may give rise todifficulty. To avoid large control signals the electrode holder may beprovided with a hydraulic amplitude amplifier as shown in FIG. 2.

The electrode holder includes two diaphragms 25 and 27 having differentsurface areas. A container 26 filled with an incompressible liquid 28 isinterposed between the diaphragms. The deflection of the piezoelectricelement 21 is transmitted to the diaphragm 25. The displacement of thisdiaphragm is transmitted by the liquid to the disphragm 27. The ratiobetween the surface areas of the diaphragms 25 and 27 determines theamplification factor of the amplitude of the deflection of theelectrode. The control voltage required for the piezo-electric elementmay be reduced by the same factor. In a practical embodiment of anapparatus according to the invention provided with a piezo-electirccontrol element the step displacement of the stepping motor was 1.25 pm.This displacement was achieved in l millisecond. The maximum controlvoltage for the piezo-electric element was 500 volts, and theamplification factor of the hydraulic amplifier was 10. At 1,000 Hertzthe impedance of the piezo-electric element was 16 kOhms. The controlelement comprised 10 plates of diameter 10 mm and thickness 1 mm. Theplates were made of a ceramic piezo-electric oxide.

What is claimed is:

1. Apparatus for removing material from an electrically conductingworkpiece by means of spark erosion, in which sparks are producedbetween the workpiece and an electrode, which apparatus includes anelectrode holder driven by a motor, characterized in that the electrodeholder includes a control element for the electrode in the form of anelectromechanical transducer which similarly to the motor is controlledby signal derived from the condition of the spark gap. I '2. Apparatusas claimed in claim 1, characterized in that the control element is apiezo-electric control element which is clamped at both ends andcomprises a plurality of plates made of a piezoelectric material whichare provided with electrically conducting coatings and are arranged oneabove the other in the direction of the electrode movement.

3. Apparatus as claimed in claim 2, characterized in that a hydraulicamplitude amplifier is provided in the electrode holder between thecontrol element and the electrode.

4. Apparatus as claimed in claim 1, characterized in that a hydraulicamplitude amplifier is provided in the electrode holder between thecontrol element and the electrode.

0 i i i l 32233?) I UNITED ST TES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 75 7 39 DatedSept. 24, 1973 Ih fl JOB HARM DEJONGH ET AL It is certified that error appears in the above-identifiedpatent:

and that said Letters Patent are hereby corrected as shown below:

Column 1, line 7, which" should read '-."I'he Column 3 line 24, V deletethe or" Claim 1, line 4', "which apparatus includes" should read 1 1--comprising-- lines 15 & 6, "characterized that the" should.

read --said--- i line 6-, "includes" should read including-- Claim 2,lines & 2, "characterized in that" should read --wherein- Claim 3, linesl & 2, "characterized in that" should read --'-further comprising-- line2, delete [is] 3293" v UNITED STATES'PATENT OFFICE v CERTIFICATE OFCORRECTION.

Patent No. 3758739 Dated S t 24 1 7g Inventor(g) AL It is certified thaterror appears in the above-identified patent and that said LettersRatent are hereby corrected as shown below:

' I I r page 2 Q 7 Insert;

Claim 5 v epp aretus for removing materiel from an electricallyconducting workpiece by means of spark erosion in which sharks ereproduced between the workr viece and an electrode, electrode holder, amotor for 7 driving seid holder, electrode carried by seid electrodeholder, and an electromechanical tranducer for controlling eeidelectrode s aid transducer being controlled by a signal derived from thecondition of the spark gap.

Signed and-sealed this 25th day of December 1973.

(SEAL) Attest;

EDWARDM.PLETCHER T,JR. RENE D. TEGTMEYER I I Attesting Officer ActingCommissioner of Patents

1. Apparatus for removing material from an electrically conductingworkpiece by means of spark erosion, in which sparks are producedbetween the workpiece and an electrode, which apparatus includes anelectrode holder driven by a motor, characterized in that the electrodeholder includes a control element for the electrode in the form of anelectromechanical transducer which similarly to the motor is controlledby signal derived from the condition of the spark gap.
 2. Apparatus asclaimed in claim 1, characterized in that the control element is apiezo-electric control element which is clamped at both ends andcomprises a plurality of plates made of a piezo-electric material whichare provided with electrically conducting coatings and are arranged oneabove the other in the direction of the electrode movement.
 3. Apparatusas claimed in claim 2, characterized in that a hydraulic amplitudeamplifier is provided in the electrode holder between the controlelement and the electrode.
 4. Apparatus as claimed in claim 1,characterized in that a hydraulic amplitude amplifier is provided in theelectrode holder between the control element and the electrode.